Magnetic structure for providing smooth uniform magnetic field distribution in traveling wave tubes



y 26, 1964 D. c. BUCK 3,134,925

MAGNETIC STRUCTURE FOR PROVIDING SMOOTH UNIFORM MAGNETIC FIELD DISTRIBUTION IN TRAVELING WAVE TUBES Filed Sept. 12, 1960 2 Sheets$heet 1 Fig.l.

WITNESSES:

INVENTOR Donlel C. Buck M mi ATTOI QNEY May 26, 964 D. c. BUCK 3,

MAGNETIC STRUCTURE FOR PROVIDING SMOOTH UNIFORM MAGNETIC FIELD DISTRIBUTION IN TRAVELING WAVE TUBES Filed Sept. 12, 1960 2 Sheets-Sheet 2 United States Patent 3,134,925 MAGNETIC STRUCTURE FGR PROVIDING SMOQTH UNIFORM MAGNETIC FEEL!) DTSTRIBUTION IN TRAVELING WAVE TUBES Daniel C. Buck, Veteran Township, Chemung County, N.Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, la., a corporation of Pennsylvania Filed Sept. 12, 196i), Ser. No. 55,493 6 Claims. (Cl. 315-35) This invention relates to apparatus including magnetic structures, and more particularly to such apparatus including traveling wave tubes wherein an electron beam is focused by a magnetic field along a relatively long path.

In a traveling wave tube, an electron beam is projected through an interaction space, generally defined by a helix. The electron beam is made to interact with an electromagnetic Wave traveling along the helix. A signal to be amplified is introduced into the interaction space by being applied to the helix and amplification occurs due to the interaction of the electromagnetic fields of the signal and the electron beam. In order to attain proper operation or" the traveling wave tube, a magnetic field is provided to restrain the electron beam in a generally cylindrical form so that it does not impinge on the helix of the transmission circuit. When a magnetic field is established in the elongated air gap between the pole pieces of a traveling wave tube apparatus, the magnetic field distribution is like that shown by the solid curve in FIG. 3. The flaring of the flux lines leaving the pole pieces causes the hump near the pole pieces. The lens action of this hump forces the electron beam to take on a scalloped cross section which produces interception of the beam by the microwave interaction structure of the tube and poor interaction between the electron beam and the interaction structure. It is also found that the magnetic field set up by the magnetic structure may deviate from an axial path due to imperfections or different strengths of the permanent magnets utilized to produce the field. In my invention by providing a re-entrant flux guide from the supporting pole pieces, one is able to substantially smooth out and provide a uniform magnetic field distribution between the pole pieces.

It is, accordingly, on object of this invention to provide an improved magnetic focusing structure for particles in those devices utilizing a long path.

It is another object to provide an improved magnetic focusing structure to maintain a uniform field distribution with an elongated air gap.

It is a further object of this invention to provide an improved magnetic focusing structure to prevent impingement of electrons on the helix of a traveling wave tube.

A more general object of this invention is to provide an improved magnetic circuit for producing magnetic fields for utilization in charged particle beam apparatus.

These and other objects of my invention will be apparent from the following description taken in accordance with the accompanying drawings throughout which like reference characters indicate like parts, which drawings form part of this application and in which:

FIGURE 1 is a sectional view of a magnetic assembly embodying my invention in conjunction with a traveling wave tube taken along the axis of the traveling wave tube;

FIG. 2 shows a transverse sectional view of FIG. 1 taken along the line II-II;

FIG. 3 is a graphical representation of the axial magnetic field distribution along the tube axis of FIG. 1;

FIG. 4 shows schematically the magnetostatic equal potential lines in a plane perpendicular to the tube axis; and

FIG. 5 shows a modification of the flux guide shown in FIGS. 1 and 2.

Referring now to FIGS. 1 and 2 of the drawing, there is shown one specific illustrative embodiment of this invcntion wherein the magnetic field for focusing the electron stream in a traveling wave tube 10 is provided by a permanent magnet assembly 12. The assembly consists of two tapered permanent bar magnets 14 and 16 positioned symmetrically with respect to the tube axis and diametrically opposite. In this type of arrangement the two magnet members 14 and 16 appear to be parts of a tubular magnet surrounding the tube 10. The permanent magnets 14 and 16 may take several different shapes determined primarily on the basis that they are able to accommodate transverse Wave guides for the input and output energy to the traveling wave tube 10. The magnets 14 and 16 have their magnetic axes symmetrically disposed around the tube axis and substantially parallel thereto. The magnets 14 and 16 are magnetized longitudinally with like poles adjacent to provide an axial magnetic field distribution. The magnets 14 and 16 may be of any suitable material such as Alnico V, an iron alloy containing aluminum, nickel, cobalt, copper and titanium. Pole pieces 18 and 20 of suitable soft magnetic material such as ingot iron advantageously abut directly against the ends of the two magnet members 14 and 16 and includes the space between the ends of the upper and lower permanent magnets 14 and 16. The pole pieces 18 and 29 are annular disc shaped and positioned transverse to the tube axis. The pole pieces 18 and 20 are each provided with a centrally located aperture with sleeve members 22 and 24 positioned respectively within the apertures of pieces 18 and 211. The sleeve members 22 and 24 are of suitable magnetic material of sufficient internal diameter and in good magnetic contact with the pole pieces 18 and 213 respectively. The sleeve members 22 and 24 are cylindrical and coaxial with the tube 10. The internal dimensions of the sleeve members 22 and 24 is sufiicient to accommodate the tube 1%.

The traveling wave tube 10, which may be of any suitable type known in the art, is inserted into the space between the permanent magnets 14 and 16 and extends over the length of the assembly. The traveling wave tube 10 comprises an envelope of glass including an enlarged portion 26 within which an electron gun assembly 28 is positioned. The other portion 35) of the envelope is tubular and contains a helix transmission circuit 32 and a collector or receiver electrode 34. The electron gun 23 directs an electron beam through the helix 32 to the collector 34. The electron path thus defined is longitudinal along the tube axis.

Input and output wave guides 36 and 38 are positioned within the end portions of the magnet assembly transverse to the axis of the traveling wave tube and in energy coupling relation with the input and output ends of the helix transmission circuit 32. The spacing between the two permanent magnets affords ready access for the wave guides 36 and 38 to the helix 32.

The pole piece 18 is transverse to the axis of the traveling wave tube 16 and is positioned between the elecron gun assembly 28 and the input wave guide 36. The pole piece 2% is positioned between the electron collector and the output Wave guide 33. A magnetic shunt $9 is provided around the permanent magnet structure.

The magnetic structure described above provides an axial magnetic field substantially as shown in curve 33 of FIG. 3. This curve is of course objectionable for the reasons previously mentioned. However, by providing suitable flux guide structure the magnetic field can be changed to that illustrated by curve 35 of FIG. 3. The flux guide assembly to provide the necessary magnetic field will be described.

Referring now to FIGS. 1 and 2, the sleeve members 22 and 24 are shown to include a plurality of rods 40 spaced symmetrically about the tube axis and extending inwardly about the adjacent wave guides 36 and 3d respectively. The rods 41) are of a soft magnetic material such as ingot iron. The rods 40 are tapered so that the cross section is reduced at the end thereof with respect to the cross sectional adjacent the sleeve. The rods 49 are parallel to the tube axis.

The curve in FIG. 3 illustrates the effect of the rods 43 which substantially eliminates the bump in the magnetic field at the ends of the magnetic structure. As can be seen, the resulting field is quite uniform.

FIG. 4 shows schematically the magnetostatic equal potential lines in a plane perpendicular to the tube axis. It should be noted that the equal potentials near the tube axis are circular giving good symmetry for beam focusing. This illustrates the smoothing properties of the magnctostatic potential.

Fine adjustment in the concentricity of the magnetic field along the tube axis can be made by mounting the rods eccentrically on the ends of the sleeve. In the specific embodiment shown, four rods are positioned about the sleeve member equally spaced which easily provides for entry of the wave guide members 36 and 38.

In FIG. 5, another embodiment of the invention is shown in which rings 42, 43, 44 and 45 of soft magnetic material are spaced with respect to sleeve 22 so as to give an effective taper in flux density. The rings may also be of different thickness to provide an effective taper. This construction will also have a smoothing etfect on the magnetic field. The rings also provide access for the waveguide 36. Again with respect to the rings 42, 43, 44 and 45, a fine adjustment is provided of the concentricity of the magnetic field along the axis by moving the rings in planes perpendicular to the axis.

Although the present invention has been shown in only a few forms, it will be obvious to those skilled in the art that it is not so limited but is susceptible to various changes and modifications without departing from the spirit and scope thereof.

I claim as my invention:

1. An electron discharge device comprising an elongated electrical wave transmission circuit, an electron gun means at one end of said circuit for projecting an electron stream lengthwise of and in coupled relationship with said circuit, an electron collecting means positioned at the other end of said circuit, means adjacent said circuit to supply a longitudinal magnetic field to focus said electron stream, said magnetic field producing means comprising a pair of pole pieces of soft magnetic material defining an air gap in which said circuit is positioned, one of said pole pieces positioned adjacent said electron gun means and the other of said pole pieces positioned adjacent said electron collecting means, permanent magnet means causing magnetic ilux to flow in said air gap from one of said pole pieces to the other of said pole pieces, flux guides of soft magnetic material extending from each of said pole pieces toward the other of said pole pieces, each of said flux guides comprising a plurality of extending rod members positioned about said transmission circuit, the cross section area of said rod members being greater adjacent said pole pieces than near the end of said rod members.

2. An electron discharge device comprising an envelope and having therein an electromagnetic wave transmission system, electron gun means adjacent one end of said system for projecting an electron stream lengthwise of and in coupled relation to said system, electron receiving means adjacent the other end of said system; means for applying a magnetic field along said system positioned exterior of said envelope to focus said electron stream, said magnetic field producing means comprising a pole piece transverse to the axis of said wave transmission system and having an aperture therein in which said envelope is positioned, and means for preventing substantial impingement of said electron stream on said transmission system comprising a sleeve member of soft magnetic matcrial positioned within each of said apertures and surrounding said envelope, each of said sleeve members including extending rod-like fingers of soft magnetic material positioned parallel to the axis of said transmission system.

3. In a device of the traveling wave tube type which includes means for producing an electron beam for flow along a given path and an elongated radio frequency propagating structure disposed adjacent said path to enable interaction between electrons of said beam and radio frequency energy propagated along said propagating structure; a permanent magnet structure surrounding said path for focusing said electron beam, said magnetic structure including two end pole pieces having an opening therein, each of said openings having a sleeve member therein, one of said pole pieces positioned adjacent one end of said propagating structure and the other pole piece positioned at the other end of said propagating structure, said pole pieces including longitudinally extending fingers for providing a uniform magnetic field.

4. In a traveling wave tube comprising an envelope having therein a conductive helix extending along a predetermined axis, means positioned at one end of said helix for producing and directing an electron beam along said axis, means positioned at the other end of said helix for receiving said electron beam, coupling means along said helix for exchange of microwave energy between said helix and an external microwave device, means positioned exterior of said envelope for focusing said electron beam, said means comprising two end pole pieces having an aperture therein and a sleeve member positioned within said aperture, said envelope positioned within said sleeve, said sleeve member including longitudinal fingers extending toward the other of said pole pieces and having a smaller dimension at the end thereof than adjacent to said sleeve member.

5. An electron discharge device comprising an electrical conductor defining an electromagnetic wave transmission circuit, an electron gun means at one end of said circuit for projecting an electron stream along said conductor, an electron receiver means adjacent the other end of said circuit, means for applying a longitudinal magnetic field to focus said electron stream, said magnetic field producing means comprising a first and a second pole piece transverse to said electron stream defining an air gap in which said circuit is positioned, said first pole piece positioned adjacent said electron gun means and said second pole piece positioned adjacent said electron receiver means, means causing magnetic flux to flow in said air gap from one of said pole pieces to the other of said pole pieces comprising longitudinally magnetized magnets positioned between said pole pieces with like poles adjacent, an input wave guide transverse to said electron stream for coupling an input signal to said circuit, said input wave guide positioned within said air gap and adjacent said first pole piece, an output waveguide transverse to said electron stream for coupling an output signal from said circuit, said output waveguide positioned within said air gap and adjacent said second pole piece, fiux guides of soft magnetic material extending from each of said pole pieces toward the other of said pole pieces, said fiux guides comprising a plurality of members positioned about said transmission circuit and spaced about said input and output waveguides, each of said input and output waveguides extending between adjacent members of said fiux guides to provide communication with said transmission circuit.

6. An electron discharge device comprising an electrical conductor defining an electromagnetic wave transmission circuit, an electron gun means at one end of said circuit for projecting an electron stream along said conductor, an electron receiver means adjacent the other end of said circuit, means for applying a longitudinal magnetic field to focus said electron stream, said magnetic field producing means comprising a first and a second pole piece transverse to said electron stream defining an air gap in which said circuit is positioned, said first pole piece positioned adjacent said electron gun means and said second pole piece positioned adjacent said electron receiver means, means causing magnetic flux to flow in said air gap from one of said pole pieces to the other of said pole pieces comprising longitudinally magnetized magnets positioned between said pole pieces with like poles adjacent, an input wave guide transverse to said electron stream for coupling an input signal to said circuit, said input Wave guide positioned within said air gap and adjacent said first pole piece, an output waveguide transverse to said electron stream for coupling an output signal from said circuit, said output waveguide positioned within said air gap and adjacent said second pole piece, a flux guide of soft magnetic material extending from each of said pole pieces toward the other of said pole pieces,

6 said flux guide comprising a plurality of spaced ring shaped members of soft magnetic material surrounding said transmission circuit and spaced to provide entry, between adjacent ring shaped members, of said input and output waveguides to said circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,259,531 Miller et a1 Oct. 21, 1941 2,822,500 Bryant Feb. 4, 1958 2,844,754 Ciofii July 22, 1958 2,922,910 Meyerer Jan. 26, 1960 FOREIGN PATENTS 831,514 Great Britain Mar. 30, 1960 1,241,269 France Aug. 8, 1960 

2. AN ELECTRON DISCHARGE DEVICE COMPRISING AN ENVELOPE AND HAVING THEREIN AN ELECTROMAGNETIC WAVE TRANSMISSION SYSTEM, ELECTRON GUN MEANS ADJACENT ONE END OF SAID SYSTEM FOR PROJECTING AN ELECTRON STREAM LENGTHWISE OF AND IN COUPLED RELATION TO SAID SYSTEM, ELECTRON RECEIVING MEANS ADJACENT THE OTHER END OF SAID SYSTEM; MEANS FOR APPLYING A MAGNETIC FIELD ALONG SAID SYSTEM POSITIONED EXTERIOR OF SAID ENVELOPE TO FOCUS SAID ELECTRON STREAM, SAID MAGNETIC FIELD PRODUCING MEANS COMPRISING A POLE PIECE TRANSVERSE TO THE AXIS OF SAID WAVE TRANSMISSION 